Two prior techniques for measuring minority carrier diffusion length are discloses in two articles, the first being an article entitled "Minority Carrier Diffusion Length In Silicon by Measurement of Steady State Surface Photovoltage", by A. M. Goodman, which appeared in a publication entitled ANSI/ASTM F391-78, at pages 770 through 776; and the second being a paper entitled "Diffusion Length Measurement by a Simple Photoresponse Technique," by H. J. Hovel, IEEE 12th Photovoltaix Specialists Conference, pages 913 through 916, 1976). The Goodman method is only applicable to samples having thicknesses greater than three times the diffusion length. In addition, the method requires relatively skilled operation of a spectrometric system, and, for best results, a least squares computation from a collection of several data points. The Hovel technique involves the determination of short circuit current response at two different wavelengths, and the derivation of ratios for different diffusion lengths and surface recombination velocities. To accomplish the Hovel method, metallic contacts, including a Schottky barrier must be applied to all samples.
It is the principal object of the present invention to provide a simplified method for the determination of diffusion lengths which uses thin slices of the semiconductor material, which requires no special electrical contacts, and which can be performed by relatively unskilled personnel.
In accordance with the present invention, a thin slice or a thick slug of semi-conductor material is placed in a sample holder which when closed, automatically makes electricl contact with the sample on both sides. Light is applied at relatively low levels, first at one wavelength and then at another wavelength, and the photovoltage and light intensity at each wavelength are measured. From the normalized ratio of the two photovoltages or the two intensities, the minority carrier diffusion length may be determined, given the two wavelengths at which the sample was illuminated, and with other factors being standardized or known.
In accordance with special features of the invention, one or more of the following techniques may be employed:
1. A special sample cell may be provided with a lower electrode for automatically coupling to one side of a sample plate of semiconductor material, and a second electrode automatically engaging the upper surface of the semiconductor sample when the lid of the sample cell is clamped into place.
2. A light chopper may be provided for improving the signal-to-noise ratio.
3. A light filter switching arrangement may be provided for interchanging filters to obtain the desired different wavelengths.
4. A photocell or thermocouple may be provided for calibration.
5. Normalization arrangements may be provided to insure that the photovoltage readings are comparable. This may be most easily accomplished by varying the intensity of the lamp, when the different filter is interposed, to obtain the same intensity level at the photocell, or the same output photovoltage.
Advantages of the new testing technique include the simplicity of the equipment, so that it may be operated by inexperienced personnel, and could be employed in line sampling during manufacture or fabrication of solar cells; and, further, the apparatus can be employed with either thick or thin samples, and needs no special electrical contacting arrangements. This is in contrast to the one prior art technique which required only thick samples and the other prior art technique mentioned hereinabove, in which specially formulated contacts including a Schottky barrier layer, were required. Incidentally, a thin sample is one having a thickness in the order of one or two minority carrier diffusion lengths, or less.
Other objects, features, and advantages of the invention will become apparent from a consideration of the following detailed description, and from the accompanying drawings.